Surprisingly, as computers grow in both power and proliferation, so too does their need to borrow and share more data with other computers. This need to exchange greater amounts of information can no longer be fully satisfied by the periodic data transfer between two computers but, rather, requires the simultaneous interconnection among a number of them, each having a particular specialization yet drawing from the specialization of the others. These interconnections are known as networks, and while they are limited in size and found in only corporate environments today, vast global geodesic networks will connect millions of islands of information tomorrow.
Communication systems which allow data transfer over telephone lines at a few hundred bits per second have been an integral part of computer systems for the past few decades. Only recently have networks capable of handling several million bits per second been widely available. Local area networks (LAN) have typically offered between 100 Kb/s and 10 Mb/s among a few hundred stations, and have been limited to a local area (a kilometer or so). One such network, Ethernet, is synchronous and operates at 10 Mb/s. Because rapid information transfer is indispensable in our highly competitive society, Ethernet is being superseded by a higher capacity network knwon as the Fiber Distributed Data Interface (FDDI) which transmits 100 Mb/s of data over each of two counter-rotating rings. FDDI can tolerate a separation of up to 2 kilometers between stations, and support and a total cable distance of 100 kilometers around a ring with 500 station attachments. FDDI possesses enough bandwidth to support up to 800 voice channels or perhaps 1-2 digitized video channels. One problem with voice or video traffic over FDDI, however, is that the network and interface are asynchronous, thereby preventing timing information from passing across the network boundaries. Although FDDI uses a Timed Token Protocol to provide both synchronous and asynchronous service, no technique has emerged as being clearly superior for clock synchronization at various stations around the ring. Whereas packet switching is possible over an asynchronous network, circuit switching requires a synchronous network and for that reason a synchronous FDDI network (FDDI-II) has been proposed. Unfortunately, FDDI and FDDI-II are incompatible, which is to say that a node adapted for FDDI-II operation cannot be part of an FDDI ring.
In an asynchronous system, each link requires its own clock. This means that each link is frequency- and phase-asynchronous vis-a-vis all other links in the ring, and that timing information cannot be recovered from the bit stream. Accordingly, the distribution of synchronous information, such as conventional telephone conversations, on the FDDI system, has certain inherent problems. One well-known solution for transmitting synchronous information over an asynchronous facility is the use of elastic storage registers to buffer the differences in the bit rate. That is, data are written into a shift register at a first bit rate and read out of the shift register at a second bit rate. When packet information is being transmitted, it makes little difference whether the read and write rates are slightly different because packets are generally limited in size and the elastic storage registers can be made as large as desired. However, when transmitting continuous synchronous data, the elastic storage registers will overflow or underflow with the undesirable result that transmitted information will either be lost (overflow condition), or that incorrect information will be created (underflow condition).
U.S. Pat. No. 4,866,704 was issued on Sep. 12, 1989 and discloses a fiber-optic voice/data network. This patent teaches a technique for synchronizing a local clock by monitoring the average fill of an elastic storage register (receiving buffer), speeding up the local clock when the average fill is increasing, and slowing the clock when the average fill is decreasing so that overflow and underflow are prevented. While this technique is useful, it requires that synchronous data be continuously present to maintain synchronization.
It is also known to distribute a reference timing signal over a separate link to each node in a network, including a ring network. However, such a technique requires the installation of a separate network just for timing--thus defeating the structural simplicity of the ring and adding to its cost. It is therefore desirable to improve upon the prior art systems for distributing timing information over an asynchronous ring.